High temperature spark plug



Jan. 11, 1966 J. E. WATSON HIGH TEMPERATURE SPARK PLUG Filed Oct. 18,1962 INVENTOR J O'H N E. WATSON ATTORNEYS Unitcd States Patent 3,229,139HIGH TEMIERATURE SPARK PLUG John E. Watson, Cleveland Heights, Ohio,assignor to the United States of America as represented by theAdministrator of the National Aeronautics and Space Administration FiledOct. 18, 1962, Ser. No. 231,604

' 1 Claim. (Cl. 313-115) (Granted under Title 35, US. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to an improved ignitor plug and, moreparticularly, to an ignitor plug that is capable of operating in anextremely hot environment.

Present day devices, such as liquid fueled rockets, operate at very hightemperatures, such as 4500 F. or even higher, and utilize ignitor plugsfor ignition. For reliability in rocket firing these ignitor plugsshould have a reasonable life expectancy even when exposed to such hightemperatures.

It has been found that the conventional spark plugs that are customarilyused to ignite the rocket propellants have an expected life that ismeasured in seconds at these high temperatures. This necessitatesfrequent replacement and may even result in ignition failure.

In order to solve the ignition problems inherent with the conventionalspark plugs, the present invention utilizes an ignitor plug having anelectrically conductive annular electrode of a temperature resistantmaterial, and this electrode encloses the base of the plug casing toshield the center electrode. The annular electrode forms a chamberadjacent the innermost end of the center electrode, and an aperture inthe annular electrode that is aligned with the center electrode providesgaseous communication between the plug exterior and the centerelectrode.

It is, therefor, an object of the present invention to provide animproved ignitor plug for high temperature operation that has a lifeexpectancy that is measured in hours or days rather than seconds.

Another object of the invention is to provide an improved ignitor plugstructure having no metallic parts exposed to direct flame impingement.

Other objects and advantages of the invention will be apparent from thedrawing and specification which follows.

The drawing is a vertical section through an ignitor plug constructed inaccordance with the present invention.

An ignitor plug shown in the drawing has a tubular metal shell or casing11 that is electrically conducting. An elongated ceramic insulator 12 ispositioned Within the longitudinal bore of the casing 11.

An annular electrode 13 is positioned at the base end of the casing 11and a conical surface 14 thereof faces toward the insulator 12. Anaperture 15 at the center of the annular electrode 13 is generallycoaxial with the bore of the casing and an elongated center electrode 16extends through the insulator 12 along the axis thereof in coaxialrelationship with the aperture 15.

The inner end of the center electrode 16 extends beyond the insulator 12to provide an cr-rposed portion 17 adjacent the annular electrode 13,and this portion has a bottom surface 18 immediately adjacent theaperture 15. The bottom surface 18 of the center electrode 16 isrecessed from the exterior surface 19 of the electrode 13 that isexposed to the extremely hot temperatures of the combustion flame. Theupper end of the center electrode 16 opposite the exposed portion 17 isthreaded at 20 to receive a terminal nut 21. A washer 22 is interposedbe- 3,229,139 Patented Jan. 11, 1966 tween the nut 21 and insulator 12in a conventional manner.

Threads 23 formed on the exterior surface of the casing 12 at the basethereof adjacent the annular electrode 13 enable the ignitor plug 10 tobe mounted in an aperture in an electrically conducting wall of acombustion chamber. When the ignitor plug 10 is so installed it is inphysical and electrical contact with the combustion chamber wall, andthe annular electrode 13 is the only part extending fromthe innersurface of this wall into the combustion chamber where the surface 19 isexposed to flame impingement from the burning gases.

According to the present invention the annular electrode 13 is made ofgraphite in the form of a disc that is rigidly mounted at the base ofthe plug 10 by crimping the bottom surface of the casing 11 adjacent thethreads 23. The aunula-r electrode 13 is able to withstand a temperaturein excess of 7000 F. and serves to shield the exposed portion 17 of thecenter electrode 16 together with the ceramic insulator 12 from the hightemperatures of the burning gases while enabling the ignitor plug 10 toproduce the required spark for igniting the propellant in the combustion chamber. This spark arcs across the gap between the cylindricalexposed portion 17 adjacent the bottom surface 18 and the annularshoulder formed by the intersection of the conical surface 14 and thecylindrical aperture 15.

As is shown in the drawing, the bottom surface 18 of the centerelectrode 16 is generally coplanar with this annular shoulder, and foroptimum spark propagation a clearance of about 0.025 inch to about 0.050inch between the exposed portion 17 and the surface of the aperture 15is required although a satisfactory spark will be produced when this gapis in the range of between about 0.01 inch and about 0.15 inch. Theexposed portion 17 of the electrode 16 is in gaseous communication withthe outer surface of the ignitor plug 10 through the cylindricalaperture 15, and this enables the spark to ignite the propellant in thecombustion chamber.

Although the preferred embodiment of the invention has been disclosedand described, it will be apparent that various structural modificationsand changes may be made to the ignitor plug 10 without departing fromthe spirit of the invention or from the scope of the subjoined claim.For example, the inwardly directed surface 14 of the annular electrode13 may be planer instead of conical as shown in the drawing. Likewise,the exterior surface 19 may be tapered to provide a thinnercross-section which would store less heat.

Additionally, the exterior surface 19 may be coated with a ceramicoxide, such as zirconium oxide. With such a coating the annularelectrode 13 can be either graphite or a refractory metal such astungsten.

While the center electrode 16 is preferably steel, a high temperaturerefractory metal, such as molybdenum or tungsten may be used. In someapplications this electrode may also be made from graphite.

What is claimed is:

An ignitor for use in a high temperature environment in excess of 4500F. in the combustion chamber of a liquid fueled rocket comprising atubular meta-l shell having means for mounting the same in the wall ofthe rocket combustion chamber, an insulator at least partially disposedwithin said shell,

a graphite annular electrode disposed at one end of said tubular shelland extending therebeyond into the rocket combustion chamber, saidannular electrode being provided with a recessed portion on the innerside thereof facing said tubular shell and having a cylindrical aperturein axial alignment with said insulator for placing the recessed portionin communication with the outer surface of said annular elect-rode inthe rocket combustion chamber, said portion remote from said insulatorbeing coplanar with said shoulder on said annular electrode and spacedtherefrom for providing a spark gap, said annular electrode serving toshield said exposed portion of said center electrode from hightemperature burning gases in the rocket combustion chamber.

-jacent said recessed portion, the end of said exposed References Citedby the Examiner, UNITED STATES PATENTS Rabe'zzana.

Fernandez 313139 X Xardell 313- 138 X Hensel u 313311 X Smithells.Skunda 313-441 X Henderson 3 133 11 GEORGE N. WESTBY, Primary Examiner.

JOHN W. HUCKERT, Examiner.

P. C. DEMEO, Assistant Examiner.

